It is often necessary to measure the existing stress or strain in concrete structures at any point in time and to determine the changes in the stress either from externally applied varying loads on the concrete, changes in temperature or moisture, or long term creep [creep is defined as the deformation in concrete occurring under, and induced by, a constant sustained stress]. For example, the concrete liner of a tunnel can have a static load from the weight of the soil and varying loads from the addition or excavations of the soil above the tunnel. Measuring concrete in situ stress or strain is not easily done because concrete has a non-uniform structure composed of aggregate of varying size and a binder material. Furthermore, concrete is a relatively stiff material which makes accurate measurements of strain difficult.
Various techniques have been employed to measure in situ stress or strain in materials. One technique includes attaching a strain gauge on mounting blocks on an in situ volume of the material to establish a baseline reading, then removing the strain gauge and making a slot in between the mounting blocks of the strain gauge on the volume of material, then replacing the strain gauge, applying pressure on the walls of the slot using a flat jack, and measuring the change in strain to establish the strain that existed in the volume of material before the slot was cut. The pressure applied in the flat jack that restores the baseline reading of the train gauge corresponds to the in situ stress.
This technique has not generally been applied to concrete. As mentioned above, concrete is a relatively stiff material and thus the amount of deformation of the material before and after the slot is cut is very small and measuring the change in strain is very difficult. A major drawback of this technique is that the removal of the gauge to cut the slot will create errors in the readings even if the gauge is attempted to be placed back in the same position. Furthermore, determining an in situ modulus of elasticity of the material with this technique is not practicable.
A second technique is to attach a strain gauge in situ on a volume of concrete, then cut a circular slot around the strain gauge to define a concrete segment. However, with this technique the gauge is not readable while cutting and the circular slot is not suitable for subsequently re-applying pressure on the segment with jacks.
In these techniques in situ stress must be determined from the measured change in strain and from a modulus of elasticity determined by laboratory methods.
Accordingly a need has arisen for measuring in situ stress or strain with more accuracy in concrete structures, as well as in situ measurements of modulus of elasticity, long term creep, or direct measurements of in situ stress caused by a change of external load.